For nonrotatably connecting the support shaft with the screws or similar slip-on elements, different shaft-to-hub connections are known.
Thus, DE-C 813 154 discloses a machine with two shafts wherein a feather key or a square profile is used as a shaft-to-hub connection. According to DE 77 01 692 U1 a multiple spline shaft is used as a shaft-to-hub connection with an integrated spline, while DE 27 50 767 A1 and EP 0 001 970 A1 describe circular shafts with three separate sunk keys with the same pitch angle, and EP 0 330 308 A1 two keys with an unequal pitch angle. What is primarily used nowadays, however, is the involute shaft according to DIN 5480.
Further, there are described and compared in “Konstruktionsbücher,” Springer-Verlag, 1984, pages 210 and 211, a variety of form-fitting shaft-to-hub connections with direct and indirect form closure with consideration of the fatigue notch factors and fatigue strength fundamental for the design of the machine. However, the hub has a constant wall thickness in these cases. In an extruder of the type wherein the screw elements or similar slip-on elements closely intermesh on the entire circumference, however, the axial profile of the slip-on element is determined by three circular arcs corresponding to the screw outside diameter, the screw core diameter and the center distance of the shafts (cf. EP 0 002 131 B1).
The most economical screw shaft is that with the greatest conveying volume and simultaneously the highest input torque. The total cross section of a screw conveyor system is limited by the diameter of the housing bore and the distance between adjacent housing bores. It must be distributed proportionately over four cross sections according to the technical requirements and possibilities as well as the demand. For transporting the product, the free conveying surface is first defined, which is determined by the outside diameter of the screw element and the flight depth. This also defines the core diameter of the screw element and the center distance relative to the adjacent support shaft. Secondly, the support shaft requires the calculated usable surface proportion for axially conducting the required shaft torque for the subsequent slip-on elements. Thirdly, the constructionally necessary surface requirement for transferring the proportionate torque for the slip-on element must be taken into account, and fourthly the remaining cross-sectional area of the slip-on element in relation to the support shaft torque for a dependable slip-on element is left.
Each support shaft, which generally has a length corresponding to at least twenty times the housing bore diameter, has a multiplicity of slip-on elements slipped thereon close together. The highest possible torque that can be transmitted to the greatest calculated usable support shaft diameter dTi crucially determines the economical use of the extruder.
The efficiency and economy of an extruder is therefore determined by highest permissible long-term torque at the same time as highest volume yield of the extruder.
It is described in “Kunststoffe” 2/2005, page 75, that the performance limiting machine element of a double-screw extruder is primarily the screw core diameter. The deeper the screw elements are cut, i.e. the greater the increase in free volume of the screw elements is, however, the smaller the remaining cross section for the screw core diameter and the associated shaft-to-hub connection will be. The problem is seen here in the tooth root strength of the DIN or a similar profile, so that an asymmetric toothing is proposed for better force transmission and stress distribution. Apart from the elaborate production of the asymmetric toothing, the performance of this extruder is also increasable further.
It is the object of the invention to substantially increase the volume yield of an extruder at equal torque.